Phylogenetic analysis of East and southern African Memecylon section Buxifolia (Melastomataceae): Insights on patterns and processes of diversification

Molecular phylogenetics is a valuable approach not only for discovering previously undetected plant diversity at the species level, but also for inferring the underlying processes of diversification. In southern Africa, two species of the woody genus Memecylon have been recognised, viz. M. natalense and M. bachmannii, with the range of M. natalense reportedly extending to northern Mozambique and southern Malawi. We investigated the evolutionary relationships of southern African Memecylon with population-level sampling and comparative sequencing of two fast-evolving DNA regions (nuclear rDNA ITS and 5′ ETS). Within M. natalense sensu lato, analyses revealed three geographically outlying populations representing distinct, semi-cryptic lineages (in forests of Maputaland and on the Soutpansberg and Mount Mulanje, respectively). Another sample from the Lindi region (southeastern Tanzania) produced an ITS sequence close to that of Kenyan M. fragrans while the ETS sequence was placed nearer to those of Mozambican M. torrei and M. natalense sensu stricto. This case of phylogenetic incongruence is taken to indicate the presence of an undescribed, homoploid hybrid species in the Rovuma region of southern Tanzania and northern Mozambique, although this hypothesis needs additional study. In KwaZulu-Natal and the Eastern Cape, rDNA spacer sequences were found to be identical or nearly so amongst samples of M. natalense and M. bachmannii, a surprising result given the clear morphological and ecological differences between these species. The minimal sequence variation would seem to indicate recent divergence or perhaps introgressive hybridisation. Further study is needed of the species boundary between M. natalense and M. bachmannii.